Migrate away from legacy generator, add basic request / response body types

[sethmlarson]

Reference: #1680

Commits:

• fbaad7d Removes the old client generator and references to it, new generator is based off the old generator and the Elasticsearch specification
• cacf718 Makes all parameters keyword-only in the type stubs. There are some documentation changes here that are incidental, the documentation in the rest-api-spec for path vs. query is different.
• 985896f Adds simple types to body, responses, and type: boolean query parameters.